1. Field of the Invention
This invention relates to a small slider unit having a slide table linearly moving by use of a rotational force generated by a screw shaft rotated by a motor.
2. Description of the Related Art
An example of conventional slider units is described with reference to FIG. 3 and FIG. 4.
As shown in FIG. 3, a rail 2 is mounted on a mounting base 1 and a slide table 3 moves along the rail 2. Specifically, C-shaped cross-section guide members 4 are provided on the two sides of the slide table 3 and slidably straddle the rail 2 to allow for the movement of the slide table 3 along the rail 2.
The slide table 3 has a hole extending therethrough in the axis direction of the rail 2. A ball nut 5 is screwed in the hole, and then is engaged with a threaded portion 6a of a screw shaft 6. When the screw shaft 6 is rotated, the rotational force moves a combination of the ball nut 5 and the slide table 3 along the rail 2.
The screw shaft includes the threaded portion 6a having a screw groove formed, and a supporting portion 6c made up of a round bar having a diameter lager than that of the threaded portion 6a, and additionally a flange 6c is interposed between the threaded portion 6a and the supporting portion 6b. 
A casing 7 having a hole through which the supporting portion 6b is secured on the mounting base 1. An angular bearing 8 is built in the casing 7. A lid member 9 is secured to an opening of the casing 7 to prevent the angular bearing 7 from falling out of the casing 7.
The angular bearing 8 has an inner ring 8a through which the supporting portion 6b of the screw shaft 6 passes. The inner ring 8a has an end adjoining the flange 6c and the other end adjoining a collar 10. The inner ring 8a of the angular bearing 8 is pressed between the collar 10 and the flange 6c by a bearing nut 11 fixedly screwing into the screw groove formed on the supporting portion 6b. In this manner, the inner ring 8a is fixedly interposed between the flange 6c and the collar 10 to inhibit the screw shaft 6 from rattling in the axis direction.
One of the opposing ends of the supporting portion 6b is coupled to the threaded portion 6a , and at the other end an interlock portion 6d of a smaller diameter than that of the supporting portion 6b is provided. The leading end of the interlock portion 6d faces a rotating shaft 12 of a motor M. The interlock portion 6d of the screw shaft 6 is connected through a coupling member 13 to the rotating shaft 12. As a result, the motor M is driven so as to rotate the rotating shaft 12, whereupon the screw shaft 6 rotates concurrently with the rotating shaft 12. Then, the rotation of the screw shaft 6 moves the slide table 3 along the rail 2.
Upon the movement of the slide table 3, a reaction acts on the screw shaft 6, so that the screw shaft 6 attempts to move in the direction opposite to the direction of movement of the slide table 3. Similarly, when any external force acts on the screw shaft 6, the screw shaft 6 may possibly move in the axis direction. If the screw shaft 6 is caused to move in the axis direction as described above, the slide tale 3 will be moved to an incorrect position.
For example, the above-described slider unit is employed as a positioning apparatus for use in the manufacturing process for semiconductor devices, the slide tale 3 on which a semiconductor is mounted is moved to various positions in which the semiconductor undergoes predetermined treatments. For this purpose, high precision is required to stop the movement of the slide table 3 in a correct position. In particular, for producing small workpieces such as semiconductors, a very high degree of precision in microns is required. Accordingly, in a small slider unit, the screw shaft 6 is inhibited from moving in the axis direction by use of the bearing nut 11 to secure the flange 6c and the collar 10 to prevent play from being created in an area between the flange 6c and the collar 10.
Such a slider unit as described above is disclosed in Japanese Patent No. 3927285, for example.
As described above, for conveying a small workpiece such as a semiconductor, the slider unit itself is required to be reduced in size. For example, the slide table 3 of the slider unit used in the manufacturing process of the semiconductor devices moves a short distance ranging from about 20 mm to about 50 mm, so that a short screw shaft 6 is sufficient for use. Also, the load-carrying capacity is not much required. Because of this, the slide tale 3 and the mounting base 1 can be reduced in size. In some slider units, the width of the slid table 3 and the mounting base 1 is reduced to about 10 mm to about 20 mm.
However, conventional slider units must have space for placing the collar 10 and the bearing nut 11 for the purpose of inhibiting the movement of the screw shaft 6 in the axis direction. In addition, another space is required for the coupling member 13 which provides connection between the rotating shaft 12 and the screw shaft 6 (interlock portion 6d). In this manner, the collar 10 and the bearing nut 11 or the coupling member 13 must be arranged in the axis direction of the screw shaft 6. As the next logical step, the length of the screw shaft 6 is increased, resulting in an increase in the length of the entire unit.
That is, in conventional slider units, even though the movement range of the slide tale 3 is very short, it is disadvantageously necessary to restrain a reduction in size of the slider unit in the longitudinal direction.